Here's a comparison of Manhattan and Halifax from Google Earth:
Central Park is much larger but Manhattan is obviously much more built up. Like I said, I think it would be nice for Halifax to have more parks but they aren't needed by the Commons - small parks distributed throughout the city would be more useful.
There's also lots of room to improve existing parks. The Public Gardens are pretty well-maintained but most of the others are not.
The Cunard site "conceptual design" has been mentioned for a while on the WDCL website. Not sure how far off development would actually be.